Electromechanical transducers are devices that exchange electrical and mechanical energy. Such transducers have acoustic applications, such as in microphone, speaker, underwater projector, hydrophone, sonar, sonic cleaning and imaging, and weaponry applications. Transducers intended for sonar applications typically use solid-state piezoelectric elements. These elements may be made from a variety of materials, such as ferroelectric ceramic lead zirconate titanate (PZT).
In sonar applications, a multiplicity of transducers are typically configured in an array. In addition to increased signal gain and reduced interference provided by an array's directivity, operational modes that produce life-like images and yield accurate estimates of contact bearing, range, and velocity are facilitated.
Underwater transducer arrays and associated acoustic signal conditioning baffles have generally been proposed. For example, U.S. Pat. No. 1,378,420, describes a pressure release surface, sonar baffle, inertia plate, and a general manner of arrangement to implement low frequency passive sonar. Similarly, U.S. Pat. No. 2,415,832 describes a high frequency transducer array employing a resonant backing absorber that conditions the acoustic signal. These construction techniques are effective, but due to resonance operation, are inherently narrowband.
Methods for reducing mutual coupling between transducers in an array have been applied. As known to those skilled in the art, reduced mutual coupling is beneficial since high inter-element coupling is known to degrade performance of arrays that are electronically steered. An example is described in U.S. Pat. No. 4,004,266.
Advances in acoustic baffle materials and construction methods have been used to reduce acoustic signal contamination from platform self-generated noise and to further condition an array's response. Examples include felt or wool loaded panels, decoupling materials like Corprene (Armstrong Company) and Sonite (Thermal Ceramics, Augusta, Ga.), specialty materials like Syntactic Acoustic Damping Material, or SADM, (Syntec Materials Inc., Springfield, Va.) and “Fibermetal”, described in U.S. Pat. No. 4,975,799, screen baffles such as described in U.S. Pat. No. 4,669,573, air-voided composite panels and compliant tube baffles, such as those described in U.S. Pat. Nos. 4,674,595 and 5,220,535 and finally, active structure baffles, such as those described in U.S. Pat. No. 5,335,209.
The aforementioned transducer arrays and baffle technologies have various advantages and disadvantages. For example, arrays employing air-voided baffles are constrained in operation to relatively shallow depths or suffer reduced performance. Arrays using inertia plates, screen baffles, resonant absorbers, or active structures typically suffer bandwidth constraints due to the construction that are often more restrictive than the limits of the transducer. Further yet, many implementations are heavy, leading to an imbalance when transducer arrays are incorporated in ship's hull applications. Added ballast (or buoyancy) is typically required to offset the transducer array's weight.
In view of the foregoing considerations, the inventors have recognized a need for low cost, conformal, lightweight, acoustic transducer arrays for various sonar applications, such as underwater collision avoidance systems.